The eukaryote genome constantly faces the threat of damage from exogenous and endogenous mutagens. Mammalian cells, therefore, have evolved an intricate network of defenses to maintain genomic stability. p53 is at the crossroads of these defense pathways. Protection of Telomeres 1(POT1)is a single stranded DNA binding component of the shelterin complex, which is essential to the formation of a telomere-specific t-loop structure and functions as a regulator of telomere length. Consistent with observations that Telomere-Related Factor-2 (TRF2) also functions in these processes, we reported that POT1 and TRF2 cooperated to protect telomeres and regulate cellular senescence and apoptosis. The siRNA knockdown of POT1 resulted in the loss of telomeric single-stranded overhangs (3' overhangs), chromosomal instability, apoptosis in breast cancer cells, and cellular senescence in normal human fibroblasts. POT1 physically interacted with TRF2 and protected against the dominant-negative TRF2-induced telomere dysfunction. We also found that a C-terminally truncated variant of human POT1 (v5) protected telomeres and prevented cellular senescence as effeciently as the full-length POT1, but in mechanistically different manners: (a) the full-length POT1, but not v5, functions through the maintenance of 3' overhangs; (b) p53 is dispensible to v5 knockdown-induced senescence; and (c) v5 functions at only a fraction of telomeres. Based on these results, we propose that human telomeres are cooperatively regulated by the functionally distinct POT1 variants. The telomere-protective variant v5 is preferentially expressed in mismatch repair-deficient tumors, which have stable chromosomes, suggesting the role of v5 in chromosome stability in human cancers. We are continuing to study p53-mediated replicative senescence modulated by POT1 or p53 isoforms that govern telomere attrition.